If my memory does not fail, you can compile it dynamically by typing wmake libso inside the source folder (wherever you choose to place it). After that you have to set properly your fvSchemes dictionary (search for some examples in the forum) and include in your controlDict something like this:

Hi all, just to let you know...
I finally was able to find a good solution with the unstructured mesh. The best choice for my cases was the pointLinear scheme suggested by Markus. The solution now is a (quite) perfect circle and the behavior during the simulation is very similar to the structured case.
The drop is not perfectly still in the middle of the domain but is slightly moving in a sort of circle (probably due to the parasite current) and the solution varies slightly around the analytical one, but for the moment i'm quite happy.